Sensitizing non-metallic substrates to the deposition of electroless metal, e.g., Group IB and VIII metals, i.e., copper, cobalt, nickel, gold, silver and the like, is a key step in the production of decorative and industrially useful metallized objects, such as name plates, dials, printed circuits, and the like. This sensitization is conventionally carried out by treating the substrate either stepwise with precious metal, e.g., palladium or platinum, ions in solution followed by a solution of stannous tin or similar ions, or all in one step with a unitary colloidal suspension of precious metal or with a soluble complex of precious metal ion, stannous ion and an anion. This produces a sensitive surface which when immersed in a conventional electroless metal deposition bath causes metal to deposit on all of the sensitized areas thereof.
A number of proposals have been made to carry out such processes more economically and efficiently:
Chiecchi, U.S. Pat. No. 3,379,556 discloses immersion in a beta resorcylato chromic chloride solution to eliminate pretreatments such as sealing, sandblasting, etching and the like. This method still requires the use of a two step, stannous-palladium subsequent treatment, see, for example, Schneble, Jr. et al, U.S. Pat. No. 3,403,035 and U.S. Pat. No. 3,033,703. Moreover, the Werner-type chromium complexes are difficult to prepare, stabilize and use. In addition, the complex must be polymerized after application and before subsequent treatment steps.
Bernhardt et al, U.S. Pat. No. 3,547,784 discloses treating a non-metallic surface with stannous salt then with a silver salt and then electrolessly plating using processes and deposition baths for copper, nickel and silver found, for example, in Schneble, Jr. et al, U.S. Pat. Nos. 3,527,215 and 3,347,724. The Bernhardt et al process is conventional and the point of novelty resides in using a particular copolymer of vinyl chloride, which was not easy to metallize up until the time of the invention.
In a more recent development, there have been provided the so-called metal reduction sensitizers, which can employ base metal ions, followed by treatment with reducing solutions or radiant energy, e.g., heat, light and the like, to produce the sensitized surface.
The metal reduction sensitizing process consists of coating a surface, preferably one which has been activated in known ways either to render it permanently polarized and wettable, or microporous, with a reducible metal salt solution, e.g., CuSO.sub.4.5H.sub.2 O, NiSO.sub.4.6H.sub.2 O, and the like, then either draining, semi-drying or completely drying the so-treated surface. Sensitization is then completed by immersing the surface into a strongly reducing medium, e.g., a sodium borohydride solution, during which step the metal salts are reduced to elemental metal particles. This sensitized surface is then rinsed and electrolessly plated.
Because thorough rinsing of excess metal salts before transferring the workpiece to the reducing medium cannot be practiced, there are numberous difficulties when dealing with copper clad surfaces (on other portions of the article). Rinsing usually removes all of the metal salts. Moreover, there is also the problem of drag-over of excess metal salts into the reducing medium, and this shortens its life and also turns it black with atomic metal particles.
Obviously, if a means could be provided to rinse excess and unwanted metal salts from the surface before immersion in the reducing medium, the above noted problems would be avoided. In addition, control will be facilitated because rinsing will provide a positive indication that only those final catalytic elemental particles which remain are those which are adsorbed by the surface.
According to the present invention, improvements are provided in the wettability and adsorptivity of metal compound media which are used to render surfaces sensitive to electroless metal depositions. Because of their improved wettability and adsorptivity, such compositions can be formulated from base metals or combinations thereof with precious metals which have performance comparable to traditional expensive and somewhat unstable entirely precious metal-based sensitizers.
In comparison with the prior art techniques, the instant system provides the following distinct advantages:
(i) more complete rinsing between the first medium and any subsequent transformation agent can now be tolerated because of tremendously improved adsorption of any metallic compound or element in the first medium to the surface;
(ii) "take" or coverage in the electroless metal bath is wholly uniform and rapid; and
(iii) in the case of activated substrates, metallization within the surface micropores is deep and complete enhancing bond strength.